Method for pre-treating a ring support prior to the &lt;alfin&gt; process

ABSTRACT

The aim of the invention is to provide a ring support that is as oxide-free as possible, prior to being cast-in with piston material. To achieve this, the ring support is annealed in a vacuum or in a hydrogen atmosphere once the ring support has been welded to the sheet steel part.

[0001] The invention relates to a method for pre-treating a ring support for aluminum pistons prior to the “alfin” process.

[0002] In the case of aluminum pistons for diesel engines, the groove for the first piston ring is generally protected against wear by means of armorings made of austenitic cast iron, which are referred to as ring supports.

[0003] A good bond between the ring support and the base material of the piston is essential during casting-in. This bond is produced, before production of the piston, by means of a so-called ALFIN layer, a layer of iron aluminides that is formed by dipping the ring support into an AlSi melt. In order for the layer to be formed in optimal manner, and in order for a good bond between the ring support and the aluminum material to be present later, the surface of the iron material must be oxide-free.

[0004] It is known to blast the ring support before dipping, in order to remove oxides. However, this method does not always produce satisfactory results.

[0005] The invention therefore concerns itself with the problem of producing as oxide-free a surface of the ring support as possible prior to the “alfin” process.

[0006] This problem is solved by means of a method according to claim 1. Advantageous further developments are the object of the dependent claims.

[0007] Comparable heat treatment methods under a hydrogen atmosphere are known for cold-formed austenitic steels, e.g. for corrugated hoses.

[0008] A metallically shiny surface is produced by annealing in a vacuum or in a nitrogen atmosphere. The blasting that has been necessary until now can therefore be eliminated.

[0009] The annealing temperature preferably lies between 1000° C. and 1130° C. A period of 5-20 minutes at temperature is then sufficient. At lower temperatures, in the range of 900° C. to 950° C., the time period at temperature must be extended to as much as 1 hour.

[0010] The cooling speed after annealing should be at least 2 K/min and is preferably greater than 5 K/min. This prevents decomposition of the surface structure of the ring support, which is usually austenitic.

[0011] Annealing in a vacuum should take place at as low a pressure as possible, preferably 10⁻³ to 10⁻⁶ mbar.

[0012] Of course, the method can be used not only for uncooled ring supports but also for cooled ring supports, i.e. for ring supports in which direct cooling by motor oil takes place radially within the ring support back, by means of a sheet-metal cooling channel that is welded on. 

1. Method for pre-treating ring supports for aluminum pistons prior to the “alfin” process, characterized in that the ring supports are annealed in a vacuum or in a reducing hydrogen atmosphere, at temperatures between 900° C. and 1130° C.
 2. Method according to claim 1, characterized in that the annealing takes place over a time period of 5 to 10 minutes.
 3. Method according to claim 1 or 2, characterized in that after annealing, a cooling speed of >5 K/min is adjusted, in order to prevent austenite decomposition. 